1. Field of the Invention
The present invention relates to an electrooptical element such as a light modulation element having an electrooptical effect.
2. Description of the Prior Art
An electrooptical effect is such a phenomenon that when an electric field is applied to an optical medium, the refractive index of the medium changes. There are two types of the electrooptical effect, namely, a linear electrooptical effect (i.e. Pockels effect) and a secondary electrooptical effect (i.e. Kerr effect).
For instance, among known materials having the electrooptical effects, inorganic crystals having no center symmetry such as KH.sub.2 PO.sub.4 (KDP), NH.sub.4 H.sub.2 PO.sub.4 (ADP), LiNbO.sub.3, and PLZT have both Pockels effect and Kerr effect. However, the Pockels effect is generally greater than the Kerr effect in these materials. On the other hand, inorganic substances having a center symmetry such as Ba.sub.2 Na.sub.2 Nb.sub.5 O.sub.15, or fine crystal aggregates or liquids, such as ceramics having certain compositions such as PLZT, organic liquids such as nitrobenzene, acetophenone, benzyl alcohol, or carbon disulfide, and solutions prepared by dissolving polar substances such as nitrostyrene or nitroanisole into a solvent, have primarily the Kerr effect.
When these substances are used as electrooptical modulation elements, in the case of single crystal materials, the preparation of the crystals is difficult because it involves withdrawal of the crystals from an aqueous solution or from a molten solution at a high temperature. Further when the material has the Pockels effect, it is necessary to carry out electric field treatment at a temperature higher than the Curie point thereof and to divide the material into single units before use. However, while such an inorganic material has a superior hardness, it has a drawback that brittle and cracks are likely to form during or prior to or after the electric field treatment due to electric strain or temperature strain. Thus, there is a defect that it is hardly possible to obtain electrooptical material of high quality by mass production. Further, when it is subjected to electric field treatment, a piezo-electric property is imparted to the crystal. If it has a piezo-electric property, a vibration occurs when a voltage is repeatedly applied to between the electrodes, and especially when the frequency of the applied voltage coincides with the normal vibration of the element, retardation of transmission of light becomes abnormally great due to birefringence effect of stress, and thus, the property for a mudulator is thereby impaired. Further, a single crystal material has a birefringence property of the crystal itself. Therefore, it is necessary to provide a wave length plate in order to reduce the retardation of light transmission caused by the birefringence of the crystal itself. Hence there is a drawback that the optical system will inevitably become complicated.
It can be said not only with respect to a single crystal but also to ceramics such as PLZT, that an inorganic solid is generally hard and brittle and it is difficult to obtain an optically uniform solid having a large size. Accordingly, there is a drawback that the cost becomes high.
With respect to organic substances having electrooptical effects, conventional ones are either liquid by themselves or they are used in a form of a liquid, as dissolved in a suitable solvent. Accordingly, they are optically isotropic and thus has a merit that they have no birefringence when used as an electrooptical element. However, in a construction of a modulation element, they have to be sealed in a glass cell having a pair of parallel electrode plates sealed therein, and accordingly the element tends to have an extra thickness, which in turn limits the possibility of minimizing the element. Besides, there is an additional drawback that the glass cell and the electrodes are mechanically weak, and thus tend to lead to a degradation of the performance due to a deformation of breakage or to a total breakdown.
It will be considered possible to produce a superior electrooptical element having no such drawbacks as are inherent to the above mentioned known electrooptical materials made of inorganic substances or liquid organic substances, if there is available a readily formable material such as a thermoplastic resin having a high electrooptical effect. However, among known thermoplastic resins, there is not such a material which is known to have a good electrooptical effect and which is practically useful. It is known that piezoelectric films made of polymers or co-polymers composed mainly of vinyl fluoride or vinylidene fluoride exhibit great electrooptical effects. It is assumed, however, that the apparent great electrooptical effects are created by a change of the birefringence of the film having a stretch-orientation due to a strain of the film which is, in turn, created by an inverse piezo-electricity created upon application of a voltage to the film having a stretch-orientation. In fact, with a film or other formed material having no stretch-orientation and no great piezo-electricity, the electrooptical effects are extremely small. Further, the above mentioned piezo-electric films have a drawback that their electrooptical effect are very much affected by the temperature change.